1. Field of the Invention
This invention relates to a system for enabling a power control system load to recover from a power outage after power is restored.
2. Description of the Related Art
Power control systems that receive power from a power source, usually an A.C. power line, and regulate the intensity of a lighting load or the power supplied to other types of loads are well known. Since the A.C. line and other power sources are subject to interruption, such systems must accommodate such interruptions by returning to a known operating condition when the power is restored.
In simple systems, such as standard rotary and linear slide wallbox dimmers, the operating condition to which the system returns is identical to the condition before the interruption (assuming no one has adjusted the control while power was off). This occurs because the dimmer operating condition is completely determined by the position of a mechanical control element. Since this control element position is unaffected by loss of electrical power, the standard dimmer can be said to "remember" its operating condition prior to loss of power even during extended power interruptions. In this case, the memory function is provided by the mechanical friction which prevents the control element from changing its position.
In more complex systems, the condition of the system at any given time during normal operation can depend upon electrical conditions, such as the state of various logic or computer circuits, as well as mechanical conditions like the position of controls or manually operated switches. Unlike mechanical devices, the electrical condition of these circuits does not generally remain constant if electrical power is interrupted for more than a very brief period (on the order of microseconds for some circuits). Therefore, unless special circuitry is used that retains its state without electrical power, or an auxiliary source of power is provided, information about the condition of a control system is lost when a power interruption occurs. In this case, the system lacks a memory function.
Even in a system that has a memory of limited duration, as is the case if an auxiliary power source to sustain the memory is derived from a battery, a charged capacitor, or the like, there is a loss of memory if the interruption is of sufficient duration to exhaust the auxiliary power source.
In systems that lack a memory function, the designer must provide for the condition which the system will assume upon restoration of power. In lighting control systems, one common provision is for the system to assume the "off" condition, such that little or no power is being applied to the load. Such a control system is exemplified by the Neptune series of light dimmers, manufactured by Lightolier.
Even systems that have a memory function may be designed to return to a predetermined condition, independent of the system's condition before the power interruption. One such system is the Orion.RTM. lighting control, manufactured by Lutron Electronics Co., Inc. Although the Orion control has full memory of its condition before a power interruption, it may be designed to return to predetermined settings, independent of its original condition. For example, one recovery mode for the Orion system is a gradual increase in load power up to a predetermined value.
In all cases where a system returns to a predetermined condition after a power failure and recover, it remains in that condition until some input is received from the user to indicate what state it should be in. Once the user input is received--for example, by manipulation of a switch or control--the system changes from the predetermined condition to the new condition requested by the user and thereby resumes normal operation.
However, this prior art recovery method, where the predetermined condition is maintained until there is user input can cause problems in a lighting control system. As an example, consider a system that returns to the "off" condition, and assume it is installed to control the lighting in a public space. When power fails, all the normal lights go out and, usually, emergency lighting is activated. This allows sufficient light for security and safe movement of the people in the space. However, when power returns, the emergency lighting automatically extinguishes. Since the control system returns from a power outage in the "off" state, the normal lights do not come back on, and the space may be plunged into darkness, with a possibility of distress or injury for the people in the space.
Alternatively, if the system returns to full "on", there is no safety problem. However, if the lights are off and there is then a short power outage, the system will turn the lights full on, where they may remain for a long period of time, wasting energy, if no one is present to notice the lights on and to turn them off.